The term cluster refers to the grouping together of elements within a domain - usually spatial. See:

Physics

• In astrophysics and astrometry: star cluster, galaxy cluster
• In molecular physics and solid state physics: A collaboration of (mostly equal) atoms, halfway between molecules and crystals; see cluster (physics)
• In music: tone cluster
• In statistics: cluster sampling or cluster analysis
• metal cluster
• In economics:
  • Porter's cluster
  • Techno cluster
• In space sciences:
  • An ESA mission to study the magnetosphere: Cluster mission
  • A special class of satellites: cluster satellites

Biology

• In biomedicine:
  • cancer cluster
• In beekeeping: winter cluster
  • cell cluster

Other

• Cluster bomb
• Cluster (band)
• In computers:
  • computer cluster
  • cluster (programming), a programming language used around 1990 with amiga computers
  • file system
• In writing systems: Consonant cluster